The invention provides a sewing machine automatic thread trimming mechanism. The mechanism includes a lower shaft, a thread trimming drive cam, a moving cutter assembly, a crankshaft, a thread trimming drive assembly, a thread trimming crank and a thread trimming connecting rod, wherein the thread trimming drive cam is fixed on the lower shaft, the moving cutter assembly can swing around the lower shaft and includes at least one moving cutter and a moving cutter frame, the thread trimming drive assembly is installed at one end of the crankshaft, and the thread trimming crank is fixed at the other end of the crankshaft; the thread trimming drive assembly includes a first extending arm, the end portion of the first extending arm is connected with a rotor, the two ends of the thread trimming connecting rod are hinged to the thread trimming crank and the moving cutter frame respectively, the distance between the crankshaft and the hinged point of the thread trimming crank and the thread trimming connecting rod is L1, the distance between the lower shaft and the hinged point of the moving cutter frame and the thread trimming connecting rod is L2, the ratio of the L1 to the L2 is not smaller than 2.5, and the transmission angle gamma of the thread trimming connecting rod relative to moving cutter frame is not smaller than 40 degrees; during thread trimming, the rotor is matched with a thread trimming contact surface on the periphery of the thread trimming drive cam. The structure and transmission chains of the sewing machine automatic thread cutting mechanism are simplified, the number of complicated parts in an original mechanism is reduced, the cost is reduced, and the assembling time is reduced.